Vehicle fuel systems typically have a fuel tank for holding a supply of liquid fuel for delivery to an engine. The fuel system also includes a lift pump for pumping fuel from the tank to the engine. Some engines, particularly those that have fuel injection, also have an injection pump which raises fuel pressure beyond that produced by the lift pump so that the fuel may be injected at high pressure into the combustion chambers.
Fuel filters are included in fuel systems to remove contaminants such as dirt and water from the fuel before it reaches the engine. Fuel filters are positioned in the fuel system either on the vacuum side or the pressure side of the lift pump. Each position for the fuel filter in relation to the lift pump has advantages and drawbacks.
If the fuel filter is positioned on the vacuum side of the lift pump, it has the advantage of removing contaminants before the fuel reaches the lift pump. This protects the lift pump from contaminants and prolongs its life. The drawback associated with this position of the fuel filter is that the amount of pressure available to push fuel through the filter is limited to atmospheric pressure. As a result, severe contamination may block flow and starve other fuel system components causing damage. Loss of flow may also occur in cold temperatures when wax or ice crystals form in petroleum fuels and block flow through the filter. Fuel heaters are needed in these situations to minimize the risk of problems.
A further problem associated with placing the fuel filter on the vacuum side of a lift pump is that lift pumps sometimes fail and generate contaminants. If these contaminants are carried downstream in the fuel they may cause damage to the high pressure/fuel metering pump or the fuel injectors.
An alternative fuel system configuration is to place the filter assembly on the pressure side of the lift pump. In this position more pressure is available to push fuel through the filter which reduces the risk of fuel starvation. Also, the heat energy imparted to the fuel by the lift pump tends to make fuels flow easier. In this position the high pressure pump and fuel injectors are protected from damaging debris from failure of the lift pump.
The drawbacks of positioning the fuel filter on the pressure side of the lift pump is that the lift pump is exposed to the contaminants which may shorten its life. Further, the pumping action tends to entrain the contaminants in the fuel which makes it more difficult to filter them out when they eventually reach the fuel filter assembly.
Thus, there exists a need for a fuel filter assembly that provides better filtration while reducing the drawbacks associated with positioning the filter on only the upstream or downstream sides of the lift pump.